Analysis of spark paths in air using three-dimensional image capturing andprocessing

The tortuous paths of sparks in air need capturing in three dimensions in o rder to analyse their shapes and, ultimately, to gain a greater understandi ng of the process by which sparks and, indeed, lightning propagate. For sho rt gaps (45 mm), the method used was to obtain two orthogonal images by mea ns of a 45 degrees /45 degrees /90 degrees prism acid then to record them w ith a video camera, but for larger gaps (100-400 mm) the two orthogonal ima ges had to be obtained by means of two vertical plane mirrors with an angle of 135 degrees between them. The images from individual video frames were filtered by application of a threshold, removal of isolated points and thin ning. Finally the images were combined and reduced to a single small file c onsisting of the xyz coordinates of the L entre-line of the path. As examples of the use of these path data, a brief explanation is given of the analysis of some 700 of these image files to yield the fractal dimensio n of each path, and the relationship of one section of path to the previous section; this was determined as (a) a probability distribution for its dir ection and (b) its most probable direction with respect to that of the prev ious section.